In recent years, there have been significant advances in laser technology including solid state laser technology. This technology is being used in numerous applications including scientific research, manufacturing, medicine, security, communications, and many others. Many laser applications require reliable high power outputs and numerous mechanisms have been developed to achieve such. Indeed, high power laser amplifiers have been developed in a variety of forms to accommodate a wide variety of applications, but each amplifier generally functions to amplify the power of an input pulse or beam to provide an amplified output pulse or beam.
Design of laser amplifiers may be limited by factors related to optical component tolerances in the amplifier to pulses of high-energy laser light and average power loading. For example, in one large class of amplifier designs, known as regenerative amplifiers, multiple passes through a single gain medium or plural gain media are used for efficient extraction of gain. In these regenerative amplifiers, an optical path is defined around which an input pulse makes a number of passes before being coupled out as an amplified pulse.
Regenerative amplifiers are generally used to provide relatively strong amplification of laser pulses. Multiple passes through the gain medium are achieved by placing the gain medium in an optical resonator, together with an optical switch that may be formed by an electro-optic modulator and one or more polarizers. The number of round trips in the resonator is controlled with the optical switch and, when this number is large, a high overall amplification factor (i.e., gain) can be achieved.
While these regenerative amplifiers provide significant gain, they are often difficult to implement due to their complex cavities. Moreover, switching of the laser pulses within the cavity usually decreases the achievable pulse energies or beam due to nonlinear propagation effects within the switch. Additionally, regenerative amplifiers, under some operating conditions, suffer from chaotic or multi-stable operation. In these operating conditions, the residual gain in the gain medium after energy extraction significantly affects the gain that can be imparted on a subsequent signal pulse. Regenerative amplifiers also generally amplify one pulse at a time. Therefore, the repetition rate of the laser system is less than the reciprocal of the time between the injection and ejection of each amplified pulse.
Another concern in high power laser systems regards heat. High average power laser systems typically produce heat and most laser architectures are designed with this in mind because high temperatures within a gain medium can reduce gain and/or cause thermo-mechanical strains that result in stress-induced birefringence. Temperature distributions in a gain medium can also lead to phase distortions which result in poor laser beam quality. At more extreme temperatures, damage to the gain material or optical coatings can occur. High temperatures within a laser system can also lead to mechanical movements that result in optical misalignment.
In order to efficiently extract heat from a solid gain medium, the fraction of cooled surface area per heated volume is generally maximized. Two geometries that have been used for high power laser systems include fibers and thin disks. In the case of fibers, two dimensions are minimized and one dimension is maximized so that a large surface area to volume ratio can be obtained for efficient cooling. Additionally, fiber amplifier geometries support very large gain-length products. Fiber amplifiers, however, are not optimal for high-power short pulses because high intensities may result from transmission of laser pulse energy through a very small cross section of an optical fiber. For high pulse powers and small cross-sections, the peak intensity can be extremely high and lead to deleterious nonlinear propagation effects.
One solution for thermal management of pulsed laser operation includes using disk shaped gain media, in which only one dimension is minimized. This geometry also provides large surface areas for heat extraction per gain volume. The thin disk is typically cooled from one side and is accessed optically from the other. A reflective surface is provided between the cooling system and the gain medium. Alternatively, the disk may be cooled from both sides if a thermally conductive medium is used that can also transmit the laser pump and signal beams of energy. Generally, the signal beam is the desired beam of laser energy that is operated on and/or used in a particular application whereas the pump beam may include any type of energy operable to stimulate the gain medium and amplify the signal beam.
Though the thin disk geometry can be efficiently cooled, the pump absorption-length product and signal gain-length product for typical gain media is small. For this reason, thin disk based lasers generally use a multi-pass optical system for optical pumping, and a multi-pass system for the signal beam. After each pass of the pump beam, a portion of the pump beam is absorbed. However, after multiple passes of the pump beam, most of the pump laser light can be absorbed by the crystal. For example, small gains may be achieved each time the signal traverses through a thin disk. Higher amplitude gains thus require multiple traversals of the signal beam through the thin disk so that a larger effective gain-length product can be achieved. Because greater energy extraction can be achieved by using a larger gain volume, a thin disk may utilize a relatively large pump and signal beam spot size. This allows for higher power without exceeding intensity thresholds for deleterious nonlinear propagation effects.
Although heat can be removed from a thin disk, thermal gradients typically lead to phase distortions due to temperature dependent indices of refraction, thermal expansion, and/or strain effects. For thin disk geometries where a reflective surface is included, thermo-mechanical distortions can lead to especially strong phase distortions and each pass of the signal beam can lead to accumulated phase distortions. These phase distortions can also include birefringent effects, such as strain-induced birefringence.
Accumulated phase distortions in a laser system can also lead to “hot spots” or losses that limit the laser powers that can be achieved. For example, if a thin disk imparts focusing phase distortions to a signal beam, the beam may eventually be focused to such a small spot size that it could damage an optical component with extreme intensities or even ionize the air, losing energy in the process. Accordingly, the laser power would be intentionally “dialed down” to avoid these effects. A signal beam that accumulates a divergence with traversals of the disk can also become too large for an optic, aperture, or an amplified region for a subsequent traversal through the gain medium. Consequently, divergence also results in an energy loss that limits the laser power.
In high power diode pumped laser systems, laser energy from multiple laser diodes may be collected and transmitted to a gain region via fiber. However, such generally requires beam combining mechanisms where multiple diode sources can be independently transmitted to the gain region. This allows for higher pump powers to be used than could be obtained from a single diode source. Additionally, multimode fibers may be used to supply higher average power. In any case, fibers are limited in the amount of power that they can easily transmit. A laser system that provides for simplified beam combining from multiple sources, at the gain medium, would provide a significant benefit.